Surveillance systems are used for continuous detection and tracking of signals emitted by a signal source in a region of space under surveillance. The signals are received by an antenna array directed towards the space under surveillance and processed to determine signal parameters such as frequency and azimuth direction. The signal source may be an active stationary or moving transmitter that transmits EM radiation, such as radio transmitter, wireless telephone and so on. It may also be a passive source corresponding to a reflected signal, such as a signal that originates from an active transmitter and is picked up and subsequently reflected by a radar antenna or any other radiation source. So far as the present invention is concerned, it is immaterial whether the signal source is active or passive. The signals received by the receiving antenna array are processed so as to distinguish genuine signals from noise and to determine the frequency, amplitude and direction of each detected signal.
A problem with hitherto proposed detection systems is that detection of weak signals is difficult, thus imposing severe restrictions on the maximum distance from the antenna array for which signal detection is possible. Moreover, known systems are unable to distinguish between signals of identical frequency originating from signal sources that are spatial disposed in different directions.